The present invention relates to semiconductor laser diodes, including in particular broad area single emitter (BASE) laser diodes of high light output power which are mounted junction side down on a carrier or submount. High output power in this context means laser diodes with at least 100 mW output. Such laser diodes are commonly used in opto-electronics and industrial applications. Light output power and stability of such laser diodes are of crucial interest and any degradation during normal use is a significant disadvantage. The present invention concerns an improved design of such laser diodes, the improvement in particular significantly minimizing or avoiding degradation of such laser diodes at very high light output powers by controlling the current flow in the laser diode in a defined way.
Semiconductor laser diodes of the type mentioned above have, for example, become important components in the technology of optical communication, particularly because such laser diodes can be used for fiber pumping and other high power laser diode applications. This allows to design all-optical fiber communication systems, avoiding any complicated conversion of the signals to be transmitted, which improves speed as well as reliability within such systems. Other uses of such high power laser diodes include cable TV (CATV) amplifiers, printing applications, and medical applications.
A typical semiconductor laser diode, e.g., an AlGaAs ridge waveguide laser diode consists of a (strained) quantum well active region sandwiched by two AlGaAs cladding layers. The first cladding layer, which is grown first onto the substrate, is commonly referred to as the lower cladding layer, and is typically n-type doped. The second cladding layer, which is grown second onto the substrate, after growth of the active region, is commonly referred to as the upper cladding layer and is typically p-type doped. The entire semiconductor epitaxial structure is grown on a GaAs substrate. A first electrode metallization provides electrical contact to the first cladding layer and a second electrode metallization provides electrical contact to the second cladding layer. Typically the first electrode covers the opposite surface of the wafer from that on which the epitaxial layers are grown, and the second electrode covers at least part of the ridge waveguide. However, other doping arrangements and locations of electrodes are also possible.
Generally, such a semiconductor laser diode can be operated in two different modes. Firstly, the device can be soldered with the first electrode onto a carrier or submount, which is referred to as a junction-side-up mounted laser diode (wherein the device is soldered to the carrier or submount with the substrate surface away from the carrier or submount). Typically, narrow-stripe (single-mode) lasers with a ridge width of a couple of microns are soldered in this way. Secondly, the device can be turned upside down and soldered with the second electrode onto a carrier or submount, which is referred to as a junction-side-down mounted laser diode. Typically broad area (multi-mode) lasers, BASE, with a ridge width of the order of 100-200 μm are soldered in that way. It should be noted that this invention may be preferably applied to junction-side-down mounted BASE laser diodes. However, it should be clear that the invention is in no way limited to such devices. In particular, the invention is not limited to ridge waveguide lasers as described above, but applicable to other designs of semiconductor laser diode, for example such as a buried heterostructure laser diode.
One of the major problems of all semiconductor laser diodes is the degradation in the end section area, particularly in the vicinity of the laser diode's front facet. This degradation is believed to be caused by uncontrolled temperature increase in the facet regions (or end sections) of the ridge waveguide, especially at high power outputs. The temperature increase may be caused by unwanted carrier recombination in these regions and heating due to free carrier injection.
The local current in the end section of the laser diode's ridge waveguide, and other parts of the laser diode, is generated by the injection current driving the laser diode. Thus, to reduce the local current density and to finally prevent current flow within the laser diode's end sections, and thus the unwanted carrier recombination, it is known to reduce the current injected into these end sections. Various designs for reducing the current injected into the end sections have been tested and described, including contact lift-off, removing the contact by etching, or otherwise interrupting the contact around these regions. Some of the tested and realized designs failed due to material, processing, or reliability problems, some show undesirable side effects, and some are just impractical or too difficult to implement.
Some known ways to prevent the above described carrier recombination in the laser diode's facet regions shall be described in the following.
One attempt is disclosed in Itaya et al. U.S. Pat. No. 5,343,486. It shows a compound semiconductor laser diode with a current blocking region formed in one facet portion of the laser diode structure. However, disadvantageously this design increases manufacturing complexity. Furthermore this approach would be unsuitable for industrial manufacturing using materials that oxidize rapidly, such as AlGaAs laser diodes, due to the rapid oxidation of the Al during processing with the method of Itaya.
Yu et al. U.S. Pat. No. 6,373,875 discloses a plurality of current-blocking layers, one each over each of the end sections of the laser diode's ridge waveguide and two separate blocking layers fully covering the remaining body right and left of the ridge waveguide. This structure thus has several layers which are laterally non-contiguous and the interruption just at the edge of the waveguide may lead to undesired effects.
Sagawa et al. U.S. Pat. No. 5,844,931 discloses a “windowed” current-blocking layer covering the ridge of a ridge waveguide laser diode and the whole body with a longitudinal opening, i.e. a window, over the center part of the ridge. Apart from the fact that some of the current blocking layers in this USP are actually conductive, not isolation layers, it discloses one single layer fully covering the laser diode body, with just a window over part of the ridge. Thus, the blocking layer is longitudinally not limited to the end section(s) of the laser diode. Also it seems that the manufacturing of such a windowed blocking layer process requires very careful alignment, especially of the window, to obtain the desired results and thus appears rather complex.
A rather successful approach is an “isolation layer” process to achieve the desired unpumped end sections in a ridge waveguide laser diode. This approach differs from earlier ones in the way that an additional thin isolation layer is placed between the semiconductor contact layer and the metal contact at the laser diode end sections. The semiconductor contact layer may even be removed. Such a design is disclosed by Schmidt at al. U.S. Pat. No. 6,782,024, assigned to the assignee of the present invention and incorporated herein by reference, showing a solution with so-called “unpumped end sections” by providing an isolation layer as current blocking layer of predetermined position, size, and shape between the laser diode's semiconductor material and the metallization.
Whereas, as shown above, unpumped end sections provide often successful solutions to block current flow in one or both end sections of a high power laser diode and thus prevent overheating and resulting catastrophic optical mirror damage (COMD) breakdowns, there are still occasions where this does not suffice. There appears to be one particular problem. Simulations of the spatial distribution of the injection current in high power laser diodes show a strong peak in the current density, i.e. a current spike, at the transition between the pumped and the unpumped section of the laser diode. This current spike stresses the material locally in the region concerned. Actually, material degradation in this region of laser diodes that have been operated for some time can be observed and are visible in electric beam-induced current (EBIC) signatures of the material at this very location. This effect is especially prominent at very high powers, with high injection current densities.
Thus, there remains in the art a need for another way of controlling the injection current distribution in the vicinity of a laser diode's end sections, perhaps even avoiding the current blocking layer or structure with its problematic transition area.
The present invention aims to provide a simple and reliable design for a high power laser diode, especially a BASE laser diode, which design provides a powerful stable light output under all operating conditions, but avoids the above-mentioned end section degradation. Another object is to provide an economical manufacturing method, allowing reliable mass production of such high power laser diodes without adding significantly to the complexity of the laser diode's structure.